1. Field of the Invention
The present invention relates to an optical beam scanning device that causes a light beam from a light source to scan in a main scanning direction on photoconductive surfaces of photoconductive members, and, more particularly to a technique for realizing improvement of an optical characteristic.
2. Description of the Related Art
Conventionally, there is known a technique for, in optical beam scanning devices that shapes light beams from light sources to be a predetermined sectional shape and causes the light beams to scan in a predetermined direction, changing a beam interval in a direction for holding down a color drift caused in an image forming apparatus including the optical beam scanning device when a temperature change occurs by providing, in a pre-deflection optical system, a lens or a diffractive optical element having negative power that transmits light beams from plural light sources (JP-A-2005-221870).
There is also known an optical beam scanning device in which a resin lens, one surface of which is an anamorphic refractive surface and the other surface of which has a power diffractive surface of an elliptical shape, is arranged in front of a reflecting surface of a deflector and power of the power diffractive surface is set to reduce fluctuation in a position where a beam diameter in a main scanning direction and/or a sub-scanning direction converges (a beam waist position) to substantially zero (JP-A-2006-154701).
However, in the constitution described in JP-A-2005-221870, there is a range that the technique cannot cope with depending on a combination of materials of a housing of the optical beam scanning device, a frame that positions plural photoconductive members, and a shaft that drives an intermediate transfer belt for superimposing developed images on the plural photoconductive members and a temperature distribution.
In the constitution described in JP-A-2006-154701, since a beam passes an optical axis in the sub-scanning direction, it is impossible to change a sub-scanning direction optical path according to a temperature change and correct a color drift due to thermal expansion of an image forming apparatus.